The field of the invention is magnetic resonance imaging systems and methods. More particularly, the invention relates to a system and method for magnetic resonance imaging in which two or more magnetization preparation radio frequency pulses are utilized, such as double inversion recovery magnetic resonance imaging.
When a substance such as human tissue is subjected to a uniform magnetic field, B0, applied along, for example, the z-axis of a Cartesian coordinate system, the individual magnetic moments of the spins in the tissue attempt to align with this magnetic field, B0, but precess about the field in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field, B1, that is applied in the x-y plane and that is near the Larmor frequency of the spins, the net aligned moment, Mz, may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment, Mxy. A magnetic resonance signal is emitted by the excited spins after the excitation field, B1, is terminated. This magnetic resonance signal may be received and processed to form an image or to produce a spectrum.
The magnetic resonance signals acquired with an MRI system are signal samples in Fourier space, or what is often referred to in the art as “k-space.” Typically, a region to be imaged is scanned by a sequence of measurement cycles in which magnetic field gradients vary according to the particular localization method being used. Each measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. This is accomplished by employing magnetic field gradients (Gx, Gy, and Gz) that have the same direction as the polarizing magnetic field, B0, but which have a gradient along the respective x, y, and z axes. By controlling the strength of these gradients during each measurement cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting magnetic resonance signals can be identified. The acquisition of the magnetic resonance signal samples is referred to as sampling k-space, and a scan is completed when enough measurement cycles are performed to adequately sample k-space. The resulting set of received magnetic resonance signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Inversion recovery (“IR”) techniques are commonly used in MRI to either increase signal contrast between tissues, or otherwise null the signals originating from particular tissue types. IR acquisitions use the application of an inversion recovery radio frequency (“RF”) pulse before the application of an excitation RF pulse and subsequent data acquisition. The duration of time between the IR RF pulse and the excitation RF pulse is referred to as the inversion time (“TI”). The contrast of IR techniques is modulated by the T1 (spin-lattice) relaxation time. The TI is selected before the MRI scan begins and determines the image contrast. Signal nulling with IR techniques is performed by selecting the TI that corresponds to the time point with roughly zero longitudinal magnetization.
As an example, IR techniques are commonly performed in brain imaging applications in order to significantly reduce, or otherwise null, the magnetic resonance signals attributable to cerebrospinal fluid (“CSF”). A double (also called dual) IR (“DIR”) technique applies two inversion recovery RF pulses in succession, such that two tissues are simultaneously nulled at a time TI2 following the second inversion recovery pulse. In the brain, DIR techniques are usually set to null signals from both white matter and CSF, producing images of gray matter. The acquisition time of whole-brain DIR images is typically on the order of 10-15 minutes, which is long in a clinical setting. Furthermore, the DIR technique is usually performed in addition to a separate single IR acquisition, which provides T1-weighted images of the brain. As such, the acquisition of both a DIR and an IR image typically requires 14-23 minutes. DIR image acquisitions, however, provide useful information, and are particularly suitable for assessing certain diseases including multiple sclerosis (“MS”), epilepsy, and plaque build-up in carotid arteries. As a result, DIR acquisitions are often included in MRI brain scans for MS patients.
It would therefore be desirable to provide systems and methods for MRI that are capable of acquiring multiple different images representative of different recovery states of magnetization in a shorter amount of scan time that can currently be achieved.